This research is aimed at obtaining the data required for the development of therapeutic applications of the ionophorous antibiotics, especially to the treatment of heart disease, as well as for applications of these compounds in the study of basic processes in cells, especially ion transport in cell membranes. The study is centered on X-537A and monensin, both of which can be given a spin label without destroying the biological function, while for X-537A the circular dichroism also provides a method for elucidating the ion-binding processes. The interactions of X-537A with alkali, alkaline earth, and some transition metal ions in methanol and in hexane have been characterized in terms of stoichiometry and equilibrium.constants, for nickel and manganous ions in methanol kinetic.parameters also have been determined, with the potentially useful finding that the rates of complexation are very different although the binding constants are very similar. The compound X2H- (formed by the interaction of the acid from HX of X-537A with its conjugate base) which has been identified as an important species in certain membranes can be formed in significant amounts in the apratic solvent propylene carbamate although not in methanol nor hexane.